Segment Information (Real Estate Assets by Segment) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Real estate assets	$ 5,365,870	$ 5,114,990
North America [Member] | Operating Segments [Member] | Carolinas Richmond [Member]
Segment Reporting Information [Line Items]
Real estate assets	491,634	482,736
North America [Member] | Operating Segments [Member] | Chicago Minneapolis [Member]
Segment Reporting Information [Line Items]
Real estate assets	557,776	565,062
North America [Member] | Operating Segments [Member] | Florida [Member]
Segment Reporting Information [Line Items]
Real estate assets	483,315	476,046
North America [Member] | Operating Segments [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Real estate assets	456,591	463,739
North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Segment Reporting Information [Line Items]
Real estate assets	1,118,802	1,234,091
North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Segment Reporting Information [Line Items]
Real estate assets	277,153	332,933
North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Segment Reporting Information [Line Items]
Real estate assets	19,352	123,093
North America [Member] | Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Real estate assets	1,769,705	1,288,208
United Kingdom [Member] | Operating Segments [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Real estate assets	$ 191,542	$ 149,082